A supporting character is a character

Fictional character

A character is the representation of a person in a narrative work of art . Derived from the ancient Greek word kharaktêr , the earliest use in English, in this sense, dates from the Restoration, although it became widely used after its appearance in Tom Jones in 1749. From this, the sense of...

 of a book

Book

A book is a set or collection of written, printed, illustrated, or blank sheets, made of hot lava, paper, parchment, or other materials, usually fastened together to hinge at one side. A single sheet within a book is called a leaf or leaflet, and each side of a leaf is called a page...

, play

Play (theatre)

A play is a form of literature written by a playwright, usually consisting of scripted dialogue between characters, intended for theatrical performance rather than just reading. There are rare dramatists, notably George Bernard Shaw, who have had little preference whether their plays were performed...

, video game, movie

Film

A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

, television

Television program

A television program , also called television show, is a segment of content which is intended to be broadcast on television. It may be a one-time production or part of a periodically recurring series...

 or radio show or other form of storytelling

Storytelling

Storytelling is the conveying of events in words, images and sounds, often by improvisation or embellishment. Stories or narratives have been shared in every culture as a means of entertainment, education, cultural preservation and in order to instill moral values...

 usually used to give added dimension to a main character, by adding a relationship with this character. Sometimes supporting characters may develop a complexity of their own, but this is usually in relation to the main character, rather than entirely independently.

In some cases, especially in ongoing material such as comic books and television series, supporting characters themselves may become main characters in a spin-off

Spin-off (media)

In media, a spin-off is a radio program, television program, video game, or any narrative work, derived from one or more already existing works, that focuses, in particular, in more detail on one aspect of that original work...

if they are sufficiently popular with fans.

Supporting characters help the plot of a story and enhance the plot.